Relationship between sediment and plankton isotope ratios (delta C-13 and delta N-15) and primary productivity in Florida lakes

Surface sediment and planktonic organic matter from Florida lakes were analyzed for delta(13)C and delta(15)N to study the relationship between stable isotope ratios and lake productivity inferred from measurements of Chi a. Stepwise regression indicates that sediment delta(13)C is a function of primary productivity, which is also suggested by a strong correlation between water column Chi a and planktonic delta(13)C (r = 0.82, P = 0.002). However, Chi a explains only 38% of the variability of sediment delta(13)C. This weak correlation is mainly attributed to the presence of nonplanktonic organic matter bearing a distinct isotopic signature in the lake sediments. Sediment and planktonic delta(13)C are also significantly correlated with water-column DIC, CO2(aq), delta(13)C of DIC, TN, and TP. Sediment and planktonic delta(15)N increase from oligotrophic to eutrophic lakes but decrease in hypereutrophic lakes. The nonlinearity of delta(15)N across the trophic gradient of study lakes may be a consequence of the relative importance of cyanobacterial N-2 fixation, but this hypothesis needs further testing.